In rubber articles, such as tires, steel cords formed by intertwining a plurality of steel filaments are widely used as a reinforcing member. In particular, a tire that is used in heavy-duty vehicles such as construction and mining vehicles is used under a large load on highly uneven, rough terrain and the like. Therefore, the steel cords that act as the reinforcing member in particular need to have high strength and durability. Hence, in such large-sized tires, steel cords with a so-called multi-twisted structure are adopted, yielded by further intertwining a plurality of strands that are each formed by intertwining a plurality of steel filaments.
In the case of a multi-twisted structure steel cord, the outermost layer sheath filaments of the core strand and the sheath strands come into rough contact with each other due to cord tension occurring when the tire is used, or due to a cut input when the tire rolls over a blunt or sharp projection. Because of stress concentrating at the point of contact, the filaments may undergo early shear failure.
As an approach for solving this problem, for example JP H11-21775 A (PTL 1) discloses that in a steel cord for reinforcing a rubber article, the steel cord having a multi-twisted structure in which 5 to 7 sheath strands having a twisted layer structure with two or more layers are arrayed and intertwined around one core strand having a twisted layer structure with two or more layers, the tensile strength of the outermost layer sheath filaments forming each strand is 3040 N/mm2 or less, and the tensile strength of all of the inner filaments excluding the outermost layer sheath filaments is 3140 N/mm2 or more. In such a steel cord, early breakage of the contact portion between the outermost layer sheath filaments of the core strand and the sheath strands is suppressed, improving the cord strength as compared to a conventional steel cord for reinforcing a rubber article.